Certain applications require a variable gain CMOS amplifier which has an exponential (linear in dB) gain control characteristic over a prescribed range. The amplifier should be able to handle reasonably large signals with reasonably low distortion and have very low input noise. An example of an application is a variable gain amplifier for preconditioning CCD signals in a camcorder, where the variable gain amplifier is used to maintain an acceptable signal level input to an analog-to-digital converter.
In one prior art approach, a standard differential pair with a variable tail current is used to drive a pair of diode-connected devices, also with variable bias current. If the bias is arranged so that the current in one pair increases as the current in the other pair decreases, the resulting voltage gain has the form: gain=sqrt ((1+x)/(1-x)). This technique is described by R. Harjani in, "A Low-Power CMOS VGA for 50 Mb/s Disk Drive Read Channels", IEEE Transactions on Circuits and Systems-II: Analog and Digital Processing, Vol. 42, No. 6, June 1995, pp. 370-376. Over a limited range, this expression is a good approximation to an exponential. However, this design has drawbacks. Because current control of transconductance is used, the gain is limited by the square root nature of the device to a fairly small range. Also, the signal path linearity is not great unless large gate-to-source voltages are used.
As to linearity over a large signal range, a linear large signal MOS transconductor is described by Z. Wang et al in "A Voltage-Controllable Linear MOS Transconductor Using Bias Offset Technique", IEEE Journal of Solid State Circuits, Vol. 25, No. 1, February 1990, pp. 315-317. An analog multiplier which utilizes a linear MOS transconductor is described by Z. Wang in "A CMOS Four-Quadrant Analog Multiplier with Single-Ended Voltage Output and Improved Temperature Performance", IEEE Journal of Solid-State Circuits, Vol. 26, No. 9, September 1991, pp. 1293-1301. The known prior art circuits do not satisfactionally meet the variable gain amplifier requirements outlined above.